1. Field of the Disclosure
The disclosure relates to an optical amplifier configured with a short, active optical fiber to emit a single mode, high peak/average power bright output, and a single mode high power fiber laser system incorporating the optical amplifier and operative to emit an ultra bright, high power single mode pulsed or continuous wave (“CW”) emission.
2. Prior Art Discussion
Fiber laser systems including Ytterbium (“Yb”), Erbium (“Er”) and other rare-earth ion-doped fibers are highly efficient, cost-effective, compact and rugged light generating and light amplifying devices. Among these, Yb and Er continuous wave (CW) and pulsed fiber laser systems dominate the industrial fiber laser market mainly due to their excellent efficiency and long term stability.
Rare-earth doped fiber lasers and amplifiers represent robust, efficient and compact optical sources capable of emitting a high quality beam of highly controlled spectral quality. The output power generated by these sources is limited, however, by parasitic nonlinear optical effects (“NLE”).
Nonlinear effects (“MILE”) include stimulated Brillouin and Raman scattering (SBS and SRS), self- and cross-phase modulation (SPM and XPM), and four-wave mixing (FWM). The common origin of these effects is                high optical intensities in the fiber core, and long path for the nonlinear interaction between the in-fiber optical beam and fiber material (e.g., silica), i.e., long fibers.These effects are observed in doped fibers integrated in both high power continuous wave and high peak power pulsed fiber laser systems. In the context of high power pulsed lasers, NLEs cause, among others, unwanted spectral broadening and distortion of the pulse temporal profile.        
Different NLEs have a number of commonalities. For example, an optical threshold power at which nonlinear effects manifest themselves is proportional to the fiber core area and inversely proportional to the fiber length. In other words, as the length of fibers increases and the core diameter decreases, the threshold power for NLEs becomes progressively lower. Accordingly, for high powers, a need always exists for large core diameters and short fiber lengths.
Optical fibers supporting propagation of light that may have a single mode are referred to as single mode (“SM”) fibers, whereas those supporting multiple transverse modes of radiation are called multimode (“MM”) fibers. SM fibers emit the highest beam quality having a Gaussian intensity shape for fibers with a step index profile.
Among multiple modes supported by the core of MM step index fibers, the most powerful fundamental mode has a profile very similar to a Gaussian. High order modes (“HOM”) are characterized by respective profiles of optical intensity which differ from a Gaussian and from one another. For a given step index, the number of transverse modes supported by a fiber is proportional to the core area. Therefore, large-core fibers tend to be multimode (“MM”) and, when modes are excited, emit a beam with the beam quality lower than that of SM fibers. The quality of the beam is critical for many industrial and scientific applications of high power fiber laser systems which include MM active fibers with large fiber core diameters. To meet the quality requirements, MM cores may be configured to support substantially only a fundamental mode.
The above-mentioned nonlinear effects (“NLE”) are extensively analyzed by the known prior art. One of the techniques providing the reduction of the number of transverse modes includes bend-loss-induced mode selection disclosed in U.S. Pat. No. 6,496,301, which is entirely incorporated herein by reference. Still another technique includes mode-matched launching between spliced directly to one another SM and MM fibers as developed by IPG Photonics Corporation. Both techniques are widely and successfully used in high power fiber laser systems (“HPFLS”). However, because of a need for increasingly higher powers, modern fiber amplifiers are dangerously close to their limits due to detrimental NLEs.
To minimize the undesirable presence of NLEs in a doped fiber, which is incorporated in HPFLSs with an output in a kW-MW range, it is necessary to configure rare earth ion-doped fibers with:                1. the smallest optimal length, which desirably approaches the length of an optical Rod—a short, straight optical component providing the undisturbed propagation of SM beam with practically no bending losses; and        2. the largest possible MFD of the fundamental mode to reduce light and therefore increase NLE thresholds.As to the best knowledge of the inventors, most currently available fiber designs used in ultra-high average and peak power fibers amplifiers cannot adequately meet the above-articulated requirements.        
The length of doped fibers also affects the quality of the fundamental mode. As the latter propagates through a meter(s)-long fiber, it tends to shift of the mode center gravity due to the bending of the MM fiber. As a result of it, the mode area is decreased. When doped long fibers are pumped, the overlap between intensity profiles of respective pump and fundamental modes, allowing the amplification of substantially only the fundamental mode, worsens. Hence HOMs, initially not amplified, start compromising the quality of the output beam, since the power lost by the fundamental mode transfers to HOMs. Accordingly, it is highly desirable that this overlap, known as the overlap integral, be as close to 1 as possible along the entire length of the fiber. Clearly, the latter is easier to realize in fibers with lengths not exceeding a few tens of centimeters.
Having established a need for optimal fiber configurations in MM HPFLSs in SM operations, the next step is to generate and absorb high power pump light in a fiber amplifier with the length measured in no more than a few tens of centimeters. One of the possibilities includes increasing a dopant concentration. But the latter cannot be inconsequentially increased above a certain level. For example, even at currently known maximum practical dopant concentration levels, double clad Yb-doped fibers at a 1060 nm wavelength typically reach a few meters. Such a fiber length, thus, creates favorable conditions for a low NLE threshold power. Furthermore, although high power MM laser diodes can be used in this configuration, it is known that their output is not sufficiently bright; yet many of the known industrial applications require highly bright beams.
An end-core pumping technique including launching SM pump light into the fiber core, of course, improves pump light brightness and absorption. However, as known to the artisan in the laser arts, even the most powerful, currently available SM laser diodes individually are not nearly sufficient for generating the desired powerful pump light for HPFLSs operative to emit MW peak and hundreds of W average power outputs.
A need therefore exists for an optical Yb-doped fiber amplifier operative to emit SM beams with average and peak powers in a kW-MW power range in about 976-1030 nm wavelength range.
Another need exists for the optical Yb-doped fiber amplifier based on a doped fiber which is no more than few centimeters long and thus thus configured to prevent generation of NLEs at low threshold powers.
Another need exists for a neodimium (“Nd”) fiber pump source operative to emit a SM bright pump signal of up to several hundred watts in the desired 910-960 nm wavelength range.
Another need exists for an end pumping arrangement including the described above Nd fiber pump source and Yb fiber amplifier.
Still another need exists the Yb fiber amplifier and SM Nd pump source which are configured so that an overlap integral between a pump mode, launched into the core end of the Yb-doped fiber, and single/fundamental modes, excited in the Yb fiber upon launching the pump light, is substantially equal to 1 along the entire length of the Yb-doped fiber.
Still another need exists for an ultra high power system configured with a booster, which has a short, straight doped fiber and capable of amplifying a signal light to a kW-MW range in the desired wavelength range, the SM pump source, which emits pump light at a λp wavelength, a seed laser which emits the signal light at a λs wavelength >λp, and a SM fiber wavelength division multiplexer (“WDM”) combining the signal and pump lights upstream from the booster.